The invention concerns a method for treatment of spent liquor from a pulp mill in order to recover the chemicals and energy contained in the liquor. Spent liquor in this context means black liquor and such spent liquors resulting from sulphite cooking of different kinds as well as from other pulp cooking processes, which contain cooking chemicals as well as organic substances dissolved from delignified material.
In the pulp process, the fibrous raw material is cooked in a cooking chemical solution, which in the sulphate process contains sodium sulphide and sodium hydroxide and in the sulphite process contains sulphite solutions of different kinds. During cooking organic compounds will dissolve from the wood material, and the most important of these is the lignin binding the wood fibres to each other. After cooking, the fibres are separated from the spent liquor formed by cooking chemicals and by the substances dissolved from the wood. In the sulphate process this liquor is called black liquor, whereas in sulphite methods its more general name is spent liquor. The dilute spent liquor existing after washing is evaporated to a dry matter content, which may be even 70-85% depending on the mill. Various cooking methods for separation of the fibres and based on organic solvents have also been presented. These differ from the sulphate and sulphite processes as regards their circulation of chemicals, among other things. To date the cooking methods based on organic solvents have not achieved a significant position in competition with the sulphate and sulphite methods, which are more efficient when using modem technology.
After the evaporation plant, the spent liquor is processed by burning it in controlled conditions in a spent liquor boiler, which is usually a soda recovery boiler when using sodium-based cooking solutions. The primary task of the soda recovery boiler is to bring about favourable conditions for collecting in such a form the inorganic chemicals contained in the spent liquor, that after regeneration they can once again be used in the cooking process. Another important task of the soda recovery boiler is to recover the chemical energy contained in the organic substance dissolved from the wood, which takes place as a normal steam boiler process. As the organic substance burns, heat is released from it and the heat is used for producing high-pressure steam for the production of electricity and low-pressure steam for process use. No soda recovery boiler is needed in connection with cooking based on organic solvents, but the circulation of chemicals takes place by distilling or by some other chemical method. The substance containing lignin, from which the cooking chemicals have been separated, can be burnt, for example, in an ordinary fluidised-bed boiler or in some other burning equipment.
The soda recovery boiler technology is a very conservative one. The burning device in question is one resembling the steam boiler with a structure and operation that have mainly remained the same over decades. Improving the reliability and increasing the capacity while keeping the old principles of operation have been important aspects in the development. The soda recovery boiler is usually the biggest and most expensive component in the pulp mill and its investment costs are approximately 15-25% of the total price of the mill. Since the composition of the spent liquor burnt in the soda recovery boiler entails problems to do with material technology, the values of steam produced in the soda recovery boiler are low compared with conventional power boilers, which results in a poor power-to-heat ratio from the viewpoint of electricity production.
Alternative solutions have been presented for replacing the soda recovery boiler, and of these the gasification of black liquor has come closest to commercial implementation. Patents FI 82494 and FI 91290 describe examples of methods for recovering chemicals and energy based on gasification of black liquor.
In FI Patent 82494, the black liquor is gasified in a pressurised gasification reactor at a temperature of 700-1300° C. using air or oxygen as the gasification medium, whereby the organic substance of the black liquor is converted entirely into gases.
The inorganic chemicals form a smelt consisting mainly of sodium carbonate and sodium sulphide. The heat needed by the reactions is produced by using oxygen at the early gasification reactor stage to burn the hydrogen and carbon monoxide obtained in the gasification. The gas is cooled, washed and used as fuel to generate steam and, if economically profitable, to produce electric energy.
In FI Patent 91290, the black liquor is gasified with the aid of air at a temperature of 800-1200° C., whereby the inorganic compounds are recovered in the melt phase as compounds that can be used in the cooking process and as energy of the organic compounds of the black liquor, which energy is mainly bound to the chemical compounds of the gas phase. The gases obtained in gasification and containing sodium compounds are conducted into a particle cooler and into a filter, from which the sodium dust is returned to the gasification device. The clean gas is taken to the gas turbine.
In spite of the great expectations on commercialisation of gasification, practice has shown that the energy efficiency of the gasification process is poorer than that of traditional soda recovery boilers, at least to date. Extra losses also relate to the conversion of energy. The product gas formed by the mixture of combustible and non-combustible gases has a relatively low thermal value. In addition, it is expensive to clean the product gas, and the usability of gasification plants is rather poor at the present time.